In security protocol analysis, the traditional choice to consider a single Dolev-Yao attacker is supported by the fact thatmodels with multiple collaborating Dolev-Yao attackers have been shown to be reducible to models with one Dolev-Yao attacker. In this paper, the authors take a fundamentally different approach and investigate the case of multiple non-collaborating attackers. After formalizing the framework for multi-attacker scenarios, they show with a case study that concurrent competitive attacks can interfere with each other. They then present a new strategy to defend security protocols, based on active exploitation of attack interference.